Seems like when the starter dragon evolutions come out, you can trade the spirits for one of their evolution materials.
Aside from that, they make pretty neat units (dual Zapan is a 4/196/1 lead for mono-water). Annihilation has a 100% drop rate, Mythical+ is reported at 40% and I'm pretty sure Mythical is near non-existent.
Grab one to evolve Plessie, grab two if you have a bunch of Pys, or grab, uh, 27 to use King Tans.
Edit: Today, I learned that there are far more options to trade for Karn than just Cottons and Blue Odins. I had, as potential candidates: Ryune, Kanna, Kaede, Scheat, Eschamali, two Dark Metatron, two Baldins, and three Gremory.
Huh.